Art of separating mixed gases



Patented Mar. 13, 1934v Unirse STATES .Pii'lllaN'-rv oI-jFlcE This invention relates to improvements in the art of separating mixed gases and is especially useful in the production of oxygen and/or nitro.

gen, either or both in a relatively pure state.

, One of the primary objects of the invention is to facilitate the starting and 'the operation of the system. In connection with this it is of particular advantage where there is no predrying of the air so that in starting up, instead-of the moisture being carried over in other parts of the apparatus, the interchangers are gotten down quickly to operating temperature so that the moisture is frozen out before it is carried over.

vAnother advantage of the arrangement isthat the operation of the system may be varied within limitsto suit particular conditions. f

I vaccomplish the foregoing, together with such other objects and advantages as may hereinafter appear, or are incident to theinvention, by means of anarrangement which is illustrated Ain the preferred form in the accompanying drawing, the gure being a diagrammatic view partly in side elevation and partly in section illustrating the preferred embodiment ,of the invention.

i Referring now to the drawing the air separation plant comprises two interchangers A and B, a liqueer C, an Iexpansion engine D, two exchangers K and L, a purifier M and a rectifying column made up of a pot E, a preliminary tray lsection F, a vaporizer section G, a main tray section H and a topltube section J.

Air compressed to the necessary operating pres- 1 sure, say 500 lb.. gage, enters the system at the air inlet 1 at, say, 30 centigrade. This compressed Aair is nearly free of carbon dioxide which has been removed in purification apparatus -not-` shown. The compressed air is saturatedwith water vapor, however, and contains oil vapors from the lubricant used in the compressor. l

With valves 2, 5 and 'I open, and valves 3, 4 and 6 closed, the compressed air passes up around the tubes in interchanger A and then up around the y tubes in interchanger B, leaving the latter through pipe 8. Valves 9, 10 and 11 should be closed and valves 12, 13 and 14 open. Then the oxygen and nitrogen separated in the rectifying column will be returning through interchanger B and not through interchanger A. Ii the valves-2, 3, 4, 5, 6, '1, 9, 10, 11, 12, 13 and 14 have just been reversed, interchanger A willbe cold andthe compressed air will be more or less cooled therein by warming up the interchanger and thawing out the accumulated ice. After the ice is completely thawed out and interchanger vA lwarmed up, the com- Abut will enter interchanger vl?, at about 30 centigrade. In interchanger B the ,compressed air will be cooled by the returning oxygen and nitrogen owing through the tubes therein to, say, minus mulated air being indicated on level gage 17. The

portion of the compressed air passing to expansion i engine D is expanded therein to, say, lb. gage and from thence passes throughpipe 18 to the lower end -of the preliminary tray section F.

The preliminary tray section F contains a number of traysarra-nged to cause the rising vapor from the expansion engine to bubble through liq- 7 uid which is flowing down from tray to tray. vWith a single -acting expansion engine as indicated in the figure, it was found that the fluctuations in pressure due to the intermittent discharge from tray to tray. Therefore, in order to reduce the fluctuations of pressure within the preliminary tray section, expansion chambers 19 andv 20 were provided with orifices at 21 and 22. YThe vrestrictions to ow, following chambers for-accumulation of pressure, reduced the iiuctuations in pressure suiiiciently to permit the liquid to iiow down from tray to tray. These chambers 1 were sumcient to prevent the liquid flowing down .80

may be made an integral part of the rectifying go column. i

Leaving the uppermost tray, the vapor passes up through a large central tube 23 in the vaporizer section to thespace underthe dome 24. The vapor then passes down through a large number of small tubes 25 in the vaporizer section and is partly condensed. The vapor portion leaves through pipe 26. The liquid portion passesy through liquid sealed tubes.27 and flows down from tray to tray through the preliminary tray stream 'of liquid with the rising vapor bubblingV through it in each tray results in the rising vaporA containing more and more nitrogen as it rises from tray to tray. 'The proper number of trays are provided to produce'nitrogen vapor, having the topmost tray about 99 percent pure. Selective liquefaction is avoided in the vaporizer section by having the vapor partially liqueiy as it passes downward through the tubes therein. so that a reflux liquid containing about 9 9 per t section. The interaction of the down flowing \00 vet nitrogen is produced. As this liquid passes downward from tray to tray, it. takes up oxygen from the rising vapor until it finally contains about 35 percent oxygen by the time it reaches pot E. At some intermediate tray, the oxygen percentage will be about 21 percent as in atmospheric air.

" As near this point as practicable, the liquefied air from liqueer C will be introduced through pipe 28 as determined by the control ofv valve 29.

The oxygen-rich liquid from pot E is throttled through pipe 30 by means of valve 31. This liquid enters the main tray section at 32 and overflows from tray to tray until it reaches the space above dome 24. From this space the liquid flows through pipe 33 to the space surrounding the tubes in the vaporizer, entering at a point about one-third the height of this section above its bottom point. The yliquid will boil by heat transfer from the condensing vapor within the tubes and the resulting vapor will rise and leave the vaporizer section through pipe 34, entering the main tray section somewhat above the dome 24. The vapor will bubble through the downflowing liquid on the trays until it reaches the inlet tray at 32.`

Here` it will be joined by any vapor formed by throttling the oxygen-rich liquid and the total y vapor will pass up through the trays above the inlet until it reaches the ytop tube section. In passingy through the tubes in this section, the vapor Will`be partly liquefied, the vapor fraction passing out through pipe 35 while the liquid fraction returns as a reflux liquid through the trays above the inlet to the main tray section and joins the oxygen-rich liquid admitted at 32.

The vaporizer and top tube sections thus produces reuxes of vapor and liquid respectively through the trays of the main tray section. The interaction between the rising vapor stream and the downflowing liquid stream results in the oxygen-nitrogen mixture throttled into the main tray section being separated into nearly pure nivtrogen vapor, say 99 percent pure, and nearly pure oxygen liquid, say 95 per cent pure.

'I'he oxygen-rich liquid entering the vaporizer section through pipe 33 is fractionated by boiling so that a portion of this liquid leaving through pipe 36 is of higher purity than the entering liquid. It is for this reason that pipe 33 is connected to a point some distance above .the bottom of the vaporizer section in order to minimize mixture of the less pure entering oxygen liquid with the purer oxygen liquid leaving through pipe 36. Pipe 36 is connected to exchanger K at a point somewhat above .the bottom. In exchanger K, further fractionationoccurs by boiling of the liquid in condensing a portion of the nitrogen vapor discharged from the preliminary tray section through pipe 26.

VThe remaining oxygen liquid passes through pipe 37 containing valve 38 into exchanger lL where it is ,completely vaporized. Most of the vapor formed passes through pipe 39 to liquefier C. Part of the vaporpasses through pipe 40 into exchanger K. The vapor formed in exchanger K, and that entering through pipe 40 are discharged though pipe 41 to augment the vapor reux in the main 'tray section. I In passing down through the' tubes in ex,- changer K, the nitrogen entering from pipe 26 is partly liquefied. The mingled liquid and va- ,v port pass through. pipe 42 to the pot 43 at .the vlower end of exchanger L. j The vapor passes upward through the tubes in exchanger L', where it is nearly all liquefied, the liqueiied portion flowing back into pot 43. I'he nitrogen liquid collecting in pot 43 is throttled by valve 44 through pipes 45 and 46 into the top tube section of the rectifying column. Here the nitrogen liquid is vaporized by imparting refrigeration to produce a reflux liquid in the main tray section, and the vapor passes through pipe 47 to the liqueer C.

The gas remaining afterthe liquefaction of nearly all the nitrogen from the preliminary rectiiication, contains large portions of neon, hydrogen, helium and other highly volatile gases in the original atmospheric air compressed and cooled. This gaseous mixture is further purified in the purifier M by bringing it into heat exchange relation `with the liquid nitrogen -after being throttled to a lower pressure through valve 44. It had been previously in heat exchange with liquid oxygen which vaporizes atan vappreciably higher temperature under the same pressure. The more volatile gases are further purified of nitrogen which is liquefied and separated in pot 48, returning to pot 43, through pipe 49. The puried rare gases are recovered through pipe 50 containing valve 51. In large plants, the refrigeration corresponding to the low temperatureof these volatile gases may be utilized by returning these gases through the liquefier and interchangers.

Instead of throttling the liquid air from liqueer C by valve 29 through pipe 28v into the preliminary tray section, this liquid air may be throttled by valve 52 through pipe 53, into the main tray section, the entering point being so selected that the composition of the liquid on the tray at that point is about the composition of the liquid admitted.

A 'oy-pass valve 54 is provided between the expansion engine exhaust pipe 18 and the return pipe 47 from the top tube section J to the liqueiier C.

Liquid level gages are vprovided as follows to indicate the liquid levels at the various points where liquids can accumulate, namely, 55 on bottom pot E, 56 on vaporizer G, 57 on exchanger L, 58 on pot 43, and 59 on top tube section J.

Two automatic regulators are provided to main- The peculiar advantages of the process and ar-v rangement shown will be brought out in describing the cooling down to operating temperatures and the normal operation.

A certain length of time will be required to cool downthe apparatus to operating temperatures. Before starting the compressor, bypass valve 54 is opened. Valve 38 should also be open and valves 31 and 44 may be open. Valves 29 and 52 should be closed.` vValves 2, 5, '7, 12, 13 and 14 may be open while valves 3, 4, 6, 9, l0 and 11 may be closedto have the compressed air pass through interchanger A Ybefore passing through interchanger B.

The air compressor is then started and the compressed air passed into inlet l after being freed of carbon-dioxidel either beforeror'after being compressed. As soon as the compressed air attains sulcient pressure. expansion engine D is atl The other tray section F. voverflowing from tray to tray,3

I this liquid air cools '15 the preliminary tray section and liquid finally accumulates in pot E, as shown on level gage 55. The vapor rising from the liquid air passes through pipe 26 to start cooling the exchangers K and L and the top tube section I J of the rectifying column.

Vim

ture.

' great advantage When some liquid has accumulated in pot E, valve 29 is partly closed and valve 52 partly opened to supply liquid air to the trays of the main tray section H. As the trays flows from tray to tray and finally accumulates in vaporizer section G and exchangers K and L. The accumulated liquid Will be indicated on level gages 56 and 57.

After considerable accumulation of liquid has occurred, valve 54 is gradually clos-ed to get the plant into normal operation. The advantage of by-pass valve 54 is and interchangers A and B act as an air liquefaction plant and quickly reach operating temperatures instead of cooling very slowly while cooling down the mass of metal in the rectifying column. Consequently, but little moisture is carried into parts of the system where it is frozen v and cannot be removed during normal operation. The rectifying column and exchangers are cooled by liquid air only which has been freed of mois- This conduces to longer operation of the plant as a whole before it becomes necessary to ice.

Another advantage necessary for the exhaust to pass through the increases the reirigerarectifying column. This tion available during cooling down so that the cooling down period is shorter than if the by-pass were not provided.

The provisionof both valves 29 and 52 is of in cooling down Vbecause it is thereby possible to fill many of the trays and accumulate liquid in both the preliminary and main tray sections `without closing by-pass 54. Without valve 52, for example, it wasfound necessary to partly close valve 54 in order to raise `the pressure in the preliminary tray section sumciently to force liquid from pot E through pipe 30 and valve 3l into the .main tray section. The provision of valve 52 increases the amount of refrigeration available during cooling down and thus reduces the length of the cooling down period.

When valve 54 is closed, the pressure in the preliminary tray section rises until liquefaction begins Within the tubes of Vaporizer G and exchangers K and L due to imparting heat to boil the liquid outside of these tubes. Equilibrium is iinally reached Where the heat transfer is just sufcient to liquefy all the air entering from the expansion engine, with the exception of that discharged by valve 51. Q

The liquid accumulating in the pot at the botthen accumulates as` fill with liquid it overthat by opening it, liqueer C shut down and thaw out to remove accumulated of the by-pass is that the 'l verylarge quantity of oxygen liquid.

tom of exchanger L is throttled through pipe 45 and valve 44 into top tube section J of 'the rectifying column. Here the liquid vaporizes around the. tubes in condensing vapor rising within the tubes. Very soon the normal reuxes are set up and the plant reaches steady operation.

During steady normal operation, the height of liquid in 'top tube section J will depend upon the temperature diierence available for heat transfer between the nitrogen vapor condensing within the tubes and the ynitrogen liquid vaporizing around the tubes. This temperature diierence will, in turn, depend upon the rectication pressure (at the rectiiier top) and the pressure under which the nitrogen liquid vaporizes. The latter pressure will depend upon the resistance encountered by the nitrogen upon returning through liqueer C and interchanger'B and connecting piping. The rectification pressure will be determined valve N. The adjustment of this valve may be changed until the height of liquid inthe top tube section J is any desired amount. it is desirable to operate at as high a level as possible in order to utilize to the greatest advantage the surface available and reduce the rectication pressure to a minimum value.

There will be occasions when it will .be desirable to dispense with the top tube section and dump the liquid nitrogen from pipe 46 directly into the main tray section. This will simpliiy'the construction of the heads of liqueer C and interchangers'A and B and of the return piping. The

automatic pressure valve N may still be used,` however, to govern the pressure of rectification in the main rectier.

During steady normal operation, the automatic pressure reducing valve P will be setto give a ow through orice Osuch as to produce the desired purities of the oxygen and nitrogen from the main tray section. If the flow is too large, the oxygen purity will be lower and the nitrogen purity higher than desired. If the flow is too small, the reverse will be true.

The liquid in pot E will be throttled through valve 31 as fast as it accumulates so as to maintain a nearly constant indication on liquid level gage 55.

As operation continues, solid particles of oil from the compressors and other impurities gather in the liquid oxygen in vaporizer G. These impurities are carried by the now through pipe 36 into exchanger K and from thence through pipe 37 into exchanger L where they accumulate. At intervals, valve 38 may be closed andthe liquid in exchanger L permitted to partially evaporate. Then the remaining liquid may be discharged through a drain pipe provided for that purpose, thereby carrying out of the system these accumulated impurities with minimum loss of oxygen liquid. Plants heretofore built have including no space which could be shut ofl at will and drained. It was,-therefore, impossible to remove the accumulated impurities Without seriously affecting the operation of the plant by removing a The amount of heating surface provided inv exchanger L is larger in comparison with the heating surface provided in vaporizer G and exchanger K than would correspond to the amount of heat required to Vaporize the quantity of oxygen to be Withdrawn from the plant. Consequently some excess vapor is always passing through pipe 40 from exchanger L to exchanger K. 'I'he oxygen vapor withdrawn through pipe 39 is, there- 150 icc l ice fore, not contaminated with any vapor from exchanger K produced from the less pure oxygen liquid therein. Y

As the amount of heating surface in the two exchangers K and L approximately equals the amount or surface provided in vaporizer G, the two exchangers make a more compact apparatus than a single exchanger would. -A single exchanger may be used, however, to liquefy the nitrogen Vapor withdrawn in the preliminary rectication.

Valve 51 is opened to maintain such a flow through it as will prevent the accumulation of highly volatile gases from reducing appreciably the heat transfer in exchange L.

After some hours of operation, interchanger B will have' its effectiveness reduced due to accumu lation of ice formed from moisture in thecompressed air. Valves 2, 3, 4, 5, 6, '7, 9, 10, 11, 12, 13

v and 14 may then be reversed. The returning oxygen and nitrogen will then pass through interchanger A instead of interchanger B and the compressed air will just'pass through interchanger B before passing through interchanger A when it will be cooled by the returning oxygen and nitrogen. In passing through interchanger B, the Warm compressed air will thaw out the accumulated ice. The resulting water will move down into pot R from which it may be blown out through valve 61. Y

While the plant may be operated by throttling the liquid air from liqueer C either through valve 29 into the preliminary tray section F on through valve 52 into the main tray section H, there are certain advantages to the latter method of operation. Any change in the rate of supply of liquid air then aiects conditions in the main tray section only, rather than in both the preliminary and main tray sections. The position of valve 31 does not have to be changed due to change in the flow from pot E. The liquid air helps in separating the nitrogen in the final rectification.

I claim:- y

1. In apparatus for separating mixed gases, in- .terclangers a liqueer and expansion engine eachy connected to receive compressed air from the interchangers, .a rectifying column having a preliminary tray section and a main tray section, means for introducing the exhaust of the expansion engine to the lower portion of the preliminary tray section, and means for introducing the liquid produced in the liquefler to either the said section or the main tray section or both.

2. In apparatus for separating mixed gases, interchangers, a rectifying column, an interposed liqueer connected to receive compressed gas from the interchangers and having return tubes connected to receive gases from the rectifier and to deliver gases tothe return tubes ofthe interchangers, an expansion engine also connected to receive compressed gas from the interchangers, an outlet line from the engine to the rectifier, and a by-pass connecting the outlet line to return tubes of the liquefier.

3. In apparatus for` separating mixed gases, interchangers, a rectifying column, an interposed liquefler connected to receive compressed gas from the interchangers and having return tubes connected to receive gases from the rectiiier and to deliver gases to. the return tubes of the interchangers, an expansion engine connected to receive'coinpressed gas and to deliver gas to the rectiiier, and means whereby the interchangers,

the liqueer and the expansion engine may be' operated to bring about operating temperatures therein without cooling down the rectifier.

' 4. In apparatus for separating mixed gases, interchangers, a liqueer and expansion engine each connectedto receive compressed air from the interchangers, a rectifying column having a preliminary tray section and a main tray section, means for introducing the exhaust of the expansion engine to the lower portion of the preliminary tray section, means for introducing the oxygenrich mixture produced in the preliminary tray section to the main tray section, and means for introducing the liquid produced in the liqueer to the main tray section at a point above the inlet of the oxygen rich mixture to the main tray section but substantially below the top of said section and where the composition of the liquid on the trays is substantially the same as the liquid so introduced.

5. In apparatus for separating` mixed gases, interchangers, a rectifying column, an interposed liquefier connected to receive compressed gas from the interchangers and having return tubes connected to receive gases from the rectier and to deliver gases to the return tubes of the interchangers, an expansion engine, an inlet line from the interchangers to the expansion engine, an outlet line from the engine to the rectifier, and a by-pass connecting the outlet Vline to the return tubes of theliquefier, the column having a preliminary tray section and a main tray section, and means for introducing the liquid pro duced in the liqueer to the tray sections.

6. In apparatus for separating mixed gases including interchangers, a rectifying column, an interposed liqueer connected to `receive compressed gases from the interchangers and having return tubes connected to receive gases from the rectier and to deliver gases to the return tubes of the inerchangers, an expansion engine conected to receive compressed gas and to deliver gas to the column, means whereby the interchangers, liqueiier and expansion engine'may be operated to bring about operating temperatures therein without cooling down the rectifier and means introducing liquid produced in the liquefier to the column.

7. In apparatus for separating mixed gases, interchangers, a rectifying column, an interposed liqueer connected to receive compressed gases from the interchangers and having return tubes connected to receive gases from the column and to deliver gases to the return tubes of the interchangers, an expansion engine, an inlet line from the interchangers to the engine, an outlet line from the engine to the rectifier, a by-pass connecting the outlet line to the return tubes of the lquefier and a valve in the by-pass.

8. In apparatus for separating mixed gases, interchangers, a rectifying column, an interposed liqueer connected to receive compressed gases from the interchangers and having return tubes connected to receive gases from the column and to deliver gases to the return tubes of the interchangers, said column having a preliminary tray section and a main tray section, an expansionv engine, an inlet line from the interchangers to the expansion engine, an outlet line from the engine to the preliminary tray section of the column,

means for introducing liquid produced in the preliminary tray section to the main tray section, an expansion chamber in said outlet line, said outlet line being provided with an orice intermediate said chamber and the column.

WIILIAM'L. DE BAUFRE. 

